The present invention relates to a transparent substrate for which there are sought the best optical properties currently required for a glazing, whether it is a glazing for a transportation vehicle or for a building. In addition to having optical properties of the highest quality, this substrate is designed to offer additional properties in the form of modification of the wettability behavior of the substrate to behavior that can be described as superhydrophobic/superoleophobic or superhydrophilic/superoleophilic, in the form of dirt-repelling, antireflective, electrical conduction and antistatic properties.
The hydrophobicity/oleophobicity property of a substrate lies in the fact that the contact angles between a liquid and this substrate are high, for example, on the order of 120° for water. The liquid then has a tendency to flow easily in the form of drops over the substrate, by simple gravity if the substrate is inclined or under the effect of aerodynamic forces in the case of a moving vehicle. This phenomenon is the expression of a rain-repelling effect. During their flow, moreover, the droplets are capable of conveying dust, insects or more or less greasy dirt of all natures, the presence of which would result in an unsightly appearance and possibly even in an impairment of visibility through the substrate. To this extent, the hydrophobic/oleophobic substrate also exhibits a dirt-repelling property.
Known hydrophobic/oleophobic agents are, for example, fluorinated chlorosilanes and fluorinated alkylsilanes such as described in European Patent Application A1 0675087. They are applied in known manner by traditional deposition methods with or without heating.
In contrast, the hydrophilicity/oleophilicity property of a substrate is manifested by small contact angles between a liquid and this substrate, for example, ranging between 0 and 5° and preferably very much smaller than 5° for water on clean glass. This property favors the formation of transparent, thin liquid films, to the detriment of fog or ice, which are composed of miniscule droplets that detract from visibility through a transparent substrate. These antifogging and anti-icing effects observed on a hydrophilic/oleophilic substrate are well known.
Numerous hydrophilic agents, especially hydroxylated substances such as poly(hydroxyalkyl (meth)acrylates), are used in known manner for this purpose in the case of transparent substrates. Moreover, certain photocatalytic compounds, such as TiO2, are used in particular in association with glass substrates, not only for their hydrophilic character after exposure to light, but also for their ability to decompose the dirt of organic origin by a radical oxidation process; the hydrophilic/oleophilic and dirt-repelling properties are then obtained simultaneously. It is known that coatings having a photocatalytic property and comprising TiO2 can be deposited from at least one titanium precursor, in solution as the case may be, by liquid-phase pyrolysis, by a sol-gel technique or else by vapor-phase pyrolysis.
According to the foregoing, the hydrophobicity/oleophobicity property is appraised quantitatively by measuring the contact angle formed in most cases by a drop of water on a given substrate. Unless otherwise indicated, this contact angle is measured for a horizontal substrate. In reality, as already mentioned hereinabove, the purpose of conferring hydrophobicity on a substrate is to influence the dynamic behavior of liquid droplets. This is true not only for substantially vertical static substrates such as the exterior glazings of buildings and glazings for shower stalls but also for glazings for transportation vehicles. In the case of a liquid drop on a substrate inclined relative to the horizontal, there are observed two different contact angles: the advancing angle and the receding angle, determined respectively at the front and rear of the drop relative to the direction of its displacement. These angles are the values reached at the limit of detachment of the drop. The difference between the advancing angle and the receding angle is known as hysteresis. A water drop having large hysteresis or a small receding angle will have difficulty in flowing over a substrate. Thus it is readily understood that effective hydrophobicity is contingent both upon a large advancing angle and small hysteresis. Finally, it is known that the size of the drop influences the advancing and receding angles.